Control of fault geometry and mechanical stratigraphy on normal fault-related folding: a case study from Inner Moray Firth Basin

Deformation in the volume surrounding normal fault zones in sedimentary sequences is often accompanied, on a range of scales, by ductile flexure of the beds, which may evolve into drag folds. These folds can be the result of various processes throughout different periods of the fault system evolution, such as fault propagation folding (Withjack et al, 1990; Schlische, 1995; Ferrill, 2012), interaction of fault segments (Childs et al, 1996; Long & Imber, 2010; Long & Imber, 2012) or frictional drag (Davis & Reynolds, 1984). All these processes are strongly influenced by the mechanical properties of the host rocks (Ferrill & Morris, 2008, Ferrill et al, 2012). Drag geometries are not always easy to discern using seismic reflection data, but their identification is critical in evaluating fault sealing capacity or reservoir connectivity. Without taking them into consideration, juxtaposition of strata can be erroneously interpreted.

Long and Imber (2010) described the spatial distribution of fault-related folding associated with a normal fault array in Inner Moray Firth basin. They showed that rotation of the strata is related to fault growth and propagation, with the largest magnitude of rotation located above or along strike from the upper and lateral fault tips.

We analyse the same 3D seismic data calibrated with wells from Inner Moray Firth basin in order to investigate the lateral and vertical variability of ductile deformation along the normal fault surfaces. 3D autotracked surfaces of key seismic reflectors with extracted dip attribute are compared with the lower resolution gridded surfaces from Long and Imber (2010). Analysis of the fault normal dip data shows that the majority of the fault related folding is localized in the shale-dominated syn-rift sequence, while the lower stratigraphic horizons corresponding with the pre-rift sandstones do not show significant rotation of seismic reflectors. The two main sequences act as different mechanical units: the competent pre-kinematic sandstones and the incompetent syn-kinematic shales on top. Interestingly, the inclination of the faults decreases while cutting through the shale-dominated sequence. This change in geometry of the fault can also contribute to folding by translation of the hanging-wall along the curved geometry of the fault. Outcrop observations from the same stratigraphic intervals are integrated in order to overcome the limits of the seismic resolution and to better understand, at different scales, the influence of mechanical stratigraphy and fault geometries on the variability of ductile deformation.